Re: Garchomp and mismagius movesets.
-Item: Life Orb
-Ability: Sand Veil
-EVs: 252 ATK / 252 SPD
Swords Dance/Stone Edge
Personally, I like to use Jolly since, with Life Orb + Swords Dance, he has high enough ATK as it is. So, why not outspeed some Pokemon? Well, you can also use Adamant, if you so desire. Also, if you want, you can replace Swords Dance for Stone Edge, to cover more types, but Earthquake/Dragon Claw/Fire Fang should suffice.
As for the last remaining 4 EVs, you can place them anywhere you want. I, personally, wouldn't care to place them in HP since Life Orb's just going to suck the life out of 'em, anyway. And, I'm sure you know that SPCL ATK would be pointless, unless you feel compelled to replace Fire Fang for Fire Blast, if you not so faithful in Fire Fang's potential.
Oh, yeah, you could also use either Choice Band or Choice Scarf, but good luck getting walled without switch wars.
-Item: Salac Berry
-EVs: 252 SPCL ATK / 252 SPD
You can pretty much place EVs anywhere, just try not to place 'em in ATK or HP. Having ATK EVs would be a total waste, and adding HP EVs would be one more HP needed to activate Salac Berry. After that, you can sink in a Destiny Bond.
Having either Thunderbolt or Energy Ball depends on what your team needs, though Thunderbolt is mostly used.